Nevada’s Lake Mead, the biggest reservoir in the West, is on track to fall below a critical threshold in 2020, according to a new forecast by the Bureau of Reclamation.

In a prediction released Wednesday, the Bureau of Reclamation, a multistate agency that manages water and power in the West, said there is a 52% probability that water levels will fall below a threshold of 1,075 feet elevation by 2020. If Lake Mead’s water levels fall below that threshold, it could trigger the first ever federal shortage declaration on the...